This invention relates to regulated deflection circuits. Known regulation systems for TV receivers which provide chassis isolation include the following:
(1) Switched Mode Power Supply (SMPS) with free running oscillator. Regulation is achieved by changing only the oscillator frequency or by changing both frequency and pulse width. PA1 (2) SMPS with constant frequency. Regulation is achieved by varying the pulse width of the power transistor or by varying the conduction angle of the switching device. Certain switched mode supplies operate as flyback converters or backwards transformers. That is, the rectifying element on the secondary side of the input transformer is in the conductive state that is opposite that of the switching element on the primary side. Flyback or backwards operated transformers require relatively tight magnetic coupling between the primary and secondary windings. Relatively expensive molded transformers are used. Also, typically, the peak voltage that the semi-conductor switching element is subjected to is relatively large. PA1 (3) SCR deflection circuit with isolation in the input transformer. An SCR regulator is located in the primary side of the input transformer. Commutation of the SCR regulator in one typical circuit is achieved via the leakage inductance of the input transformer and a capacitor located in the secondary side of the input transformer. In such an arrangement, the physical size of the transformer becomes relatively large because the commutation current must be transformed to the primary side. To commutate the SCR properly, the leakage inductance must be relatively small. Therefore, an additional input inductance may be required in the secondary side. Also, with the capacitor located on the secondary side, commutation of the regulator SCR is difficult during the start-up interval.
Commutation of the regulator SCR in another typical circuit is achieved solely by the transformed deflection commutation switch voltage. During start-up, it will be relatively difficult to turn off the regulator SCR. Also, during steady-state operation relatively large currents will flow in the input transformer.
Combining an SCR deflection circuit with a conventional or modified SMPS with a transistor as a switch provides an unsatisfactory combination of a less reliable transistor switch and a reliable thyristor deflection switch. For cost effectiveness, it is better to use only one type of switch, e.g., a thyristor type, for the different types of switching functions.